Methods and Systems for the Rapid Detection of Concealed Objects

ABSTRACT

The present invention provides for an improved scanning process having microwave arrays comprised of microwave transmitters in radiographic alignment with microwave receivers. The microwave array emits controllably directed microwave radiation toward an object under inspection. The object under inspection absorbs radiation in a manner dependent upon its metal content. The microwave radiation absorption can be used to generate a measurement of metal content. The measurement, in turn, can be used to calculate at least a portion of the volume and shape of the object under inspection. The measurement can be compared to a plurality of predefined threats. The microwave screening system is used in combination with other screening technologies, such as NQR-based screening, X-ray transmission based screening, X-ray scattered based screening, or Computed Tomography based screening.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.12/175,599, filed on Jul. 18, 2009, which is a divisional of co-pendingU.S. patent application Ser. No. 10/952,665, filed on Sep. 9, 2004,which is a continuation-in-part of co-pending U.S. patent applicationSer. No. 10/662,778, filed on Sep. 15, 2003.

FIELD OF THE INVENTION

The present invention relates generally to a microwave imaging systemthat is compatible with X-ray and Nuclear Quadrupole Resonance (NQR)based methods and systems for detection of concealed threats, and threatresolution, and more specifically to improved methods and systems, usingdual stage scanning to process luggage for faster inspection withreduced false alarm rate.

BACKGROUND OF THE INVENTION

Conventional X-ray systems produce radiographic projection images, whichare then interpreted by an operator. These radiographs are oftendifficult to interpret because objects are superimposed. A trainedoperator must study and interpret each image to render an opinion onwhether or not a target of interest, a threat, is present. With a largenumber of such radiographs to be interpreted, and with the impliedrequirement to keep the number of false alarms low, operator fatigue anddistraction can compromise detection performance.

Advanced technologies, such as dual-energy projection imaging andComputed Tomography (CT), are being used for contraband detection,beyond conventional X-ray systems. In dual-energy imaging it isattempted to measure the effective atomic numbers of materials incontainers such as luggage. However, the dual-energy method does notreadily allow for the calculation of the actual atomic number of theconcealed ‘threat’ itself, but rather yields only an average atomicnumber that represents the mix of the various items falling within theX-ray beam path, as the contents of an actual luggage is composed ofdifferent items and rarely conveniently separated. Thus dual-energyanalysis is often confounded. Even if the atomic number of an item couldbe measured, the precision of this measurement would be compromised byX-ray photon noise to the extent that many innocuous items would showthe “same” atomic number as many threat substances, and therefore theatomic number in principle cannot serve as a sufficiently specificclassifier for threat versus no threat.

In X-ray CT cross-sectional images of slices of an object arereconstructed by processing multiple attenuation measurements taken atvarious angles around an object. CT images do not suffer much from thesuper-positioning problem present in standard radiographs. However,conventional CT systems take considerable time to perform multiplescans, to capture data, and to reconstruct the images. The throughput ofCT systems is generally low. Coupled with the size and expense of CTsystems this limitation has hindered CT use in applications such asbaggage inspection where baggage throughput is an important concern. Inaddition, CT alarms on critical mass and density of a threat, but suchproperties are not unique to explosives. CT based systems suffer fromhigh false alarm rate. Any such alarm is then to be cleared or confirmedby an operator, again interpreting images, or hand searching.

Apart from X-ray imaging systems, detection systems based on X-raydiffraction, or coherent scatter are also known. Their primary purposeis not to acquire images but to obtain information about the molecularstructure of the substances an object is composed of. The so-calleddiffraction or coherent scatter signature is based on BRAGG reflection,that is the interference pattern of X-ray light, which develops whenX-rays are reflected by the molecular structure or electron densitydistribution of a substance.

Various inspection region geometries have been developed and disclosed.Kratky, in Austrian Patent No. 2003753 publishes a refined arrangementof circular concentric apertures combined with an X-ray source and apoint detector, to gain the small angle diffraction signature of anobject placed between the apertures. More recently Harding in U.S. Pat.No. 5,265,144, uses a similar geometry but replaces the point shapeddetector aperture with an annular detector configurations. Both patentsare incorporated herein by reference.

The resulting diffraction spectra can be analyzed to determine themolecular structure of the diffracting object, or at least to recognizesimilarity with any one of a number of spectra, which have previouslybeen obtained from dangerous substances.

One approach to detecting explosives in luggage was disclosed in Britishpatent No. 2,299,251 in which a device uses Bragg reflection fromcrystal structures to identify crystalline and poly-crystallinesubstances. Substances can be identified because the energy spectrumdistribution of the polychromatic radiation reflected at selected anglesis characteristic of the crystal structure of the substance reflectingthe radiation.

U.S. Pat. Nos. 4,754,469, 4,956,856, 5,008,911, 5,265,144, 5,600,700 and6,054,712 describe methods and devices for examining substances, frombiological tissues to explosives in luggage, by recording the spectra ofcoherent radiation scattered at various angles relative to an incidentbeam direction. U.S. Pat. No. 5,265,144 describes a device usingconcentric detecting rings for recording the radiation scattered atparticular angles. Each of the prior art systems and methods, however,suffer from low processing rates because the scatter interaction crosssections are relatively small and the exposure times required to obtainuseful diffraction spectra are long, in the range of seconds andminutes. For security inspections, equipment performance has to combinehigh detection sensitivity and high threat specificity with highthroughput, at the order of hundreds of bags per hour.

U.S. Pat. No. 5,182,764 discloses an apparatus for detecting concealedobjects, such as explosives, drugs, or other contraband, using CTscanning. To reduce the amount of CT scanning required, a pre-scanningapproach is disclosed. Based upon the pre-scan data, selected locationsfor CT scanning are identified and CT scanning is undertaken at theselected locations. The inventors claim the pre-scan step reduces thescanning time required for each scanned item, therefore increasingthroughput. However, the use of CT scanning is still inefficient, notthreat specific, and does not allow for rapid scanning of objects.

U.S. Pat. No. 5,642,393 discloses a multi-view X-ray inspection probethat employs X-ray radiation transmitted through or scattered from anexamined item to identify a suspicious region inside the item. Aninterface is used to receive X-ray data providing spatial informationabout the suspicious region and to provide this information to aselected material sensitive probe. The material sensitive probe, such asa coherent scatter probe, then acquires material specific informationabout the previously identified suspicious region and provides it to acomputer. The disclosed system does not, however, address criticalproblems that arise in the course of applying a scatter probe to aselected suspicious region, including the accurate identification of asuspicious region, correction of detected data, and the nature ofprocessing algorithms used.

Nuclear quadrupole resonance (NQR) is a contraband material detectiondevice, which has applications in security screening. This technologyhas shown potential for the detection of a range of materials, inparticular it is very effective for the detection of the types ofexplosives which can be the most challenging to detect using x-rays orCT machines. One potential weakness of the technique is that, withcarefully designed electromagnetic shielding, the materials which it isbeing used to detect can be rendered undetectable. This potentialproblem is mitigated by the fact that such shielding consists ofconductive (typically metal) volumes that must completely encapsulatethe item to be detected. Because the items being searched for typicallyhave a size large in comparison with most metal clutter (i.e. keys,coins, zippers, etc) the counter measure can be detected using a varietyof metal detection techniques. However, the presence of a conductiveloop around luggage means that the simplest forms of inductive metaldetector would have limited performance.

Accordingly, there is need for an improved automatic threat detectionand resolution system that captures data through an X-ray system andutilizes this data to identify threat items in a rapid, yet accurate,manner. There is also a need for determining the presence of potentialshields of explosive materials. There is additionally a need todetermine the shielding's size, volume, and position. Furthermore, thereis a need for such detection technology to operate within enclosedmetallic tunnels. Additionally, the system should provide for greateraccuracy in utilizing pre-scan data to identify an inspection region andin processing scan data.

SUMMARY OF THE INVENTION

One object of the present invention is to provide for an improvedscanning process having a first stage to pre-select the locations ofpotential threats and a second stage to accurately identify the natureof the threat. The improved scanning process increases throughput bylimiting the detailed inspection to a small fraction of the total bagvolume, and it decreases the frequency of false alarms by applyingthreat specific analysis.

Another object of the invention is to provide for improved processingtechniques performed in association with various scanning systems. Theimproved processing techniques enable the substantially automateddetection of threats and decrease the dependence on operator skill andperformance.

Another object of the invention is to provide for a method and system toscreen for relatively small amounts of threat material.

Another object of the invention is to provide for an improved method andsystem for screening for metal.

It is an object of the present invention to have a system that isrelatively immune interference from metallic clutter items which aretypical in bags and packages.

Yet another object of the present invention is to provide a system thatis compact and compatible with being built into the structures typicalfor housing NQR equipment and X-ray or CT equipment.

A further object of the present invention is to provide a conductivevolume imaging and detection system which is relatively insensitive todistortions in the image, owing largely to cross talk between differenttransmit and receive antenna pairs or microwave reflections caused bythe presence of metallic clutter or reflections from the equipmenthousing the system.

A still further object of the present invention is to provide a systemthat uses an appropriate frequency of operation such that penetration issufficient for the detection and imaging of objects within typicalpackages and bags but with a minimal amount of inaccuracies beingintroduced due to items with high dielectric loss being present within abag.

A further object of the invention is to provide a system which willprovide information which, either alone, or in conjunction with othermetal detection techniques can be used to calculate the volume of anyregion encapsulated by conductive material.

A yet further object of the present invention is to provide a microwavedetection and imaging system that can generate metal information in one,two or three axes for display. Images may be displayed for the microwaveimaging system alone or overlaid with images from different imagingtechnologies such as computed tomography x-rays or transmission x-rayimaging systems.

A further object of the invention is a system that provides3-dimensional positional information, which can be transmitted tocomplementary detection sensors that can be targeted at volumes withinan object that cannot be screened effectively using NQR.

Accordingly, one embodiment of the present invention provides anapparatus for identifying an object concealed within a container. Theseobjects may be considered threats, such as metal, an illegal drug, anexplosive material, or a weapon. One embodiment is directed toward anintegrated security scanning system, comprising a plurality of microwavearrays comprised of microwave transmitters in radiographic alignmentwith a plurality of microwave receivers, wherein said array is inphysical communication with a housing and radiation shielding inphysical communication with the housing. The microwave array emitscontrollably directed microwave radiation toward an object underinspection wherein said object under inspection absorbs radiation in amanner dependent upon its metal content. The microwave radiationabsorption can be used to generate a measurement of metal content. Themeasurement, in turn, can be used to calculate at least a portion of thevolume and shape of the object under inspection. The measurement canalso be compared to a plurality of predefined threats.

In one embodiment, if the measurement is different than a pre-definedvalue, the object under inspection can be ignored by a system operator.Alternatively, if the measurement is different than a pre-defined value,the object under inspection can be selected for additional screening,such as NQR-based screening, X-ray transmission based screening, X-rayscattered based screening, or Computed Tomography based screening.

Optionally, the measurement can be used to generate a microwave image.The microwave image can be combined with an image produced by atechnology selected from any one of NQR-based screening, X-raytransmission based screening, X-ray scattered based screening, orComputed Tomography based screening. The measurement can be used togenerate positional information of metal content in the object underinspection. The positional information of metal content can be used todirect an analysis from material specific detection technology, such asX-ray diffraction, thermal neutron analysis or pulsed fast neutronanalysis.

Optionally, the microwave transmitters and microwave receivers areconfigured in a plurality of different configurations, such as in amanner that replicates X-ray beam fan beam geometry, X-ray beam foldedarray geometry, or Computed Tomography array geometry. Optionally, themicrowave transmitters are broad beam transmit antennas and microwavereceivers are narrow band receive antennas. The broad beam transmitantennas are configured in parallel with said narrow band receiveantennas. The broad beam transmit antennas are configured in parallelwith said narrow band receive antennas and switched such that eachtransmit antenna transmits to several receive antennas. The switchingoccurs to move an illumination point around a region.

The present invention is also directed toward a method of scanning anobject comprising the steps of subjecting the object to a firstscreening system comprising microwave arrays having at least onemicrowave transmitter in radiographic alignment with at least onemicrowave receiver and subjecting the object to a second screeningsystem selected from any one of NQR-based screening, X-ray transmissionbased screening, X-ray scattered based screening, or Computed Tomographybased screening. Optionally, first screening system operates concurrentwith said second screening system or the first screening system operatesserially with respect to said second screening system.

The present invention is also directed toward an integrated securityscanning system, comprising a first screening system comprisingmicrowave arrays having at least one microwave transmitter inradiographic alignment with at least one microwave receiver and a secondscreening system selected from any one of NQR-based screening, X-raytransmission based screening, X-ray scattered based screening, orComputed Tomography based screening. Optionally, the first screeningsystem operates concurrent with said second screening system or thefirst screening system operates serially with respect to said secondscreening system.

The aforementioned and other embodiments of the present invention shallbe described in greater depth in the drawings and detailed descriptionprovided below.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will beappreciated, as they become better understood by reference to thefollowing Detailed Description when considered in connection with theaccompanying drawings, wherein:

FIG. 1 is a schematic view of one embodiment of the dual stage X-rayscanning system;

FIG. 2 is a schematic view of one embodiment of an X-ray scanning systemfor the first stage scanning system;

FIG. 3 is a schematic view of one embodiment of the first stage of theX-ray scanning system for identifying a target region;

FIG. 3 a depicts exemplary images for identifying the location of anitem within a container;

FIG. 4 is a schematic diagram of a cross-section of one embodiment of apreferred beam delivery system for use in a second stage scanningsystem;

FIG. 5 is a schematic diagram of one embodiment of the beam deliverysystem of the second stage scanning system;

FIG. 6 is a schematic diagram of an exemplary look up source fortransmission spectra;

FIG. 7 is a schematic representation of a beam delivery system havingmultiple energy dispersive detectors;

FIG. 8 is a graphical representation of an artificial neural network;

FIG. 9 is a flow diagram describing a plurality of steps for practicingone embodiment of the present invention; and

FIG. 10 is a flowchart depicting a process of training the neuralnetwork.

FIGS. 11( a), 11(b) and 11(c) are projection drawings depicting aresonator body in a preferred NQR security system as used in the presentinvention;

FIG. 11( d) depicts a perspective view of the resonator body of the NQRsecurity system in FIGS. 11( a), 11(b), and 11(c);

FIG. 12( a) illustrates the layout of an enclosed resonator probe in apreferred NQR security system as used in the present invention;

FIG. 12( b) is a drawing depicting the inspection volume or cutaway ofthe enclosed resonator probe in a preferred NQR security system as usedin the present invention;

FIG. 12( c) is a drawing illustrating the coil cross-section and showsthe magnetic flux path within the resonator body of the NQR system ofthe present invention;

FIG. 12( d) depicts the equivalent circuit diagram of an enclosedresonator probe in a preferred NQR security system as used in thepresent invention;

FIG. 13 is a drawing depicting the layout of a NQR baggage scannerhaving single resonator coil in another preferred embodiment of the NQRsystem of the present invention;

FIG. 14 is a diagram showing a single tuning mechanism for controllingtuning vanes of two or more resonator probes, in another preferredembodiment of the NQR system of the present invention;

FIG. 15 is a diagram depicting the layout of a dual coil NQR baggagescanner with single tuning mechanism, in another preferred embodiment ofthe NQR system of the present invention;

FIG. 16 illustrates pathways of transmit/receive pairs in one embodimentof a microwave imaging system of the present invention;

FIG. 17 depicts microwave antenna arrays housed in a system with acomplimentary security technology, including, but not limited to an NQRshielding waveguide, X-ray tunnel, or CT tunnel;

FIG. 18 depicts a preferred configuration of multiple antenna arrays ina two-dimensional imaging system;

FIG. 19 illustrates a combined image derived from the image of themicrowave imaging system and the image from an X-ray system, in anotherpreferred embodiment of the present invention;

FIG. 20 is an isometric sketch depicting microwave antenna arrays housedin a system with a complimentary security technology, as in FIG. 17;

FIG. 21 is a diagram showing a baggage scanner configured to comprisetwo novel resonator probes and a transmission X-ray system in anotherembodiment of the present invention;

FIG. 22 depicts one embodiment of shielding for probes that have athinned section of conductive material;

FIG. 23 illustrates a transmitted microwave fan beam and a folded arrayof detectors in a geometry typical of security line scan x-ray systems;and

FIG. 24 depicts an exemplary configuration of transmit/receive antennasused to collect data producing CT images of conductive objects.

DETAILED DESCRIPTION OF THE INVENTION I. Introduction

The methods and systems described herein are directed towards finding,locating, and confirming threat items and substances. Such threats maycomprise explosives such as C4, RDX, Semtex, Seismoplast, PE4, TNT,dynamite, PETN, ANFO among others, as well as other contraband such asdrugs. Although the embodiments have been described in the context of abaggage inspection system, it should be evident to persons of ordinaryskill in the art that items other than luggage such as other packages,mail, and cargo-containers, or even processed food stuffs, can also beanalyzed and screened or graded and that the descriptions are exemplaryand are not restrictive of the invention. Further, while the inventionis described as a dual-stage system and method, the processingtechniques discussed herein can be applied to each of the individualscanning stages.

II. An Overview of the Dual Stage System

Referring to FIG. 1, a dual stage scanning system 100 comprises ahousing 130, which encompasses a conveyor system 115 for movingcontainers, baggage, luggage, or similar object 110 through a pluralityof scanning stages 150, 155. A sensor system 165 is connected at theentrance to determine when an object being scanned 110 enters the scanfield and communicates with a controller [not shown] to activate ordeactivate an X-ray radiation source, 170, 172, as needed. A lead linedtunnel 180 surrounds the conveyor to reduce radiation leakage outsidethe equipment. At least one radiation source is not expressly depictedin FIG. 1 and would be visible if the system were viewed from theopposite side.

III. A Preferred Embodiment of the Dual Stage System of the PresentInvention

a. A Preferred First Stage

Referring to FIG. 2, the first stage 150, comprises two X-ray camerasheld together by a support structure 220, such as a frame or yoke, forstability. Each camera consists of an X-ray source 170, 171, a X-rayfocusing means, such as a collimating slit comprised of a radio-opaquematerial, for example lead (not shown), and an array of detectors, 200,201. In one embodiment, it is preferred that the detectors areconfigured into a L-shape in order to save space. One of ordinary skillin the art would appreciate that other folded configurations may beacceptable, provided that the detectors are appropriately positionedrelative to the inspection region and X-ray source.

Behind each slit collimator, a thin sheet of X-rays 210 is formed.Within the sheet, a fan of pencil beams can be defined, shown as dashedlines in FIG. 2, by connecting lines between the stationary focus, notshown, and channels in the detector array. Between focus and detector isa tunnel 180 through which the luggage is transported or moved using anymeans known in the art, including, for example, a conveyor 115, thesurface of which is depicted in FIG. 2. Wherever in the system radiationhas to be transmitted from X-ray sources 170, 171 and through the regiondefined by tunnel 180, the conveyor belt support structure as well asthe tunnel has windows constructed from materials essentiallytranslucent to X-rays. The collimating slits and detector arrays areoriented so that the radiation-fans intersect the main conveyor surfacewithin a few degrees of perpendicular relative to the conveyor surface.The two X-ray sources and their fans point in different directions.

In one preferred embodiment, the detector arrays are mounted on printedcircuit boards with a vector positioned normal to their surfacesdirected to the X-ray focus. An exemplary printed circuit board has acapacity of 64 channels, and the boards are physically arranged inVenetian blind configuration. The detector arrays consist of lineararrays of silicon photodiodes that are covered with scintillationmaterial, which produces light when exposed to X-rays. The light isdetected by the photodiodes that produce corresponding photo currentsignals. The detectors measure to what degree the X-ray signal hasattenuated due to passing through a defined inspection volume.Specifically, the detected data are converted to digital format,corrected for detector gain and offset, and then stored. The requiredprocessor means may comprise computing hardware, firmware and/orsoftware known to persons of ordinary skill in the art. When a containerunder inspection is moving through the tunnel and passing through theX-ray projections, both detector arrays are being sampled repetitivelybetween 50 and 500 times per second. Displaying the line projections ona monitor renders the projection X-ray image.

While a conventional line scan system could be used as the first stagescanning system, it is preferred to use the system as described herein.More specifically, the present invention provides for the placement ofat least two X-ray sources such that the directions of the X-rayprojections emanating from the sources are mirrored relative to thecentral vertical plane. Therefore, from the perspective of a view alongthe path of conveyance through the first stage scanning system, at leastone X-ray generator is mounted at a five o'clock position and at leastone X-ray generator is mounted at the 7 o'clock position.

One of ordinary skill in the art would appreciate that the first stagescanning system is not limited to the specific embodiments describedabove and that other variations are included within the scope of thisinvention. In one alternative embodiment, detector arrays are expandedfrom a single array to multiple parallel arrays of detectors. In asecond alternative embodiment, X-ray projections are taken usingtwo-dimensional pixelated detector planes, without requiring the use ofa conveyance means. It should be appreciated that, while the presentinvention will be further described using a description of the inventionbased on using the line scan configuration of single stationary foci andsingle line detector arrays in conjunction with a means of conveyance,the present invention includes other systems and methods that generateX-ray projection images and that such systems and methods can be used inthe novel dual stage scanning system disclosed herein.

An alternative embodiment uses dual energy imaging. Dual energy imagingcan be utilized to display an image where materials of a metallicconstituency are suppressed (not displayed) or materials of an organicconstituency are suppressed. Having the ability to selectively displaycertain materials within images helps reduce image clutter. For example,when inspecting containers for masses or explosives, which have littleor no metallic component, the “organic materials only” display ispreferred. The dual energy approach can be further refined toautomatically discriminate between similar materials of higher and lowerrelative atomic numbers, such as between a plastic comprised of morelower atomic number atoms like hydrogen and carbon and a plasticcomprised of more higher atomic number elements like oxygen andnitrogen; or between aluminum (atomic number 13) and steel (atomicnumber 26).

In one embodiment, dual energy data is generated by using an X-ray tubewith extended spectral emission, which is standard, in conjunction witharrays of stacked detectors, where the first detector is positioned todetect more of the lower energy, or so-called softer X-ray photons, andthe second detector is positioned to detect the balance of the energy,namely the higher energy, or so-called harder, photons. The seconddetector is typically positioned behind the first detector. The lowenergy and high energy measurements are combined in a suitable way usinga series of calibration measurements derived from dual energymeasurements taken of identified organic and metallic materials of knownthicknesses and result in the display of images, including organic onlyor metal only images. One of ordinary skill in the art would appreciatethat various dual energy line scan systems are commercially available.

It is preferred to use projection imaging as the first stage scanningstep in this invention. Features shown in the projection images can beused by an operator to make a final decision on whether items identifiedin a container represent a threat of some type. Additionally, by takingprojections from at least two different angles, it is possible totriangulate the location of a potential threat relative to the physicalcoordinates of the system and use those coordinates to perform a morespecific and focused second stage scan. The triangulation processlocalizes certain items that generate features of interest in the imagesand identifies their location in the form of system coordinates.

To perform the triangulation process, the images that form the basis ofthe triangulation process and that are used to identify a target regionare first identified. In one embodiment, the images are analyzed by anoperator who visually and approximately determines a plurality of X-rayimage characteristics, such as degree of attenuation and projected area,associated with mass, atomic number (identified using image colorcoding), and shape. Operators also use contextual information, such asan X-ray opaque organic mass in a transistor radio or a suspiciouslythick suitcase wall. The analytical process is known to those ofordinary skill in the art and includes the interpretation of X-ray imagecharacteristics.

In another embodiment, images are identified by determining the targetregions automatically. For example, where the screening target is a massof plastic explosive, known algorithms, working on dual energy X-rayprojection image data, can be combined to automatically find suchtarget. Examples for such algorithm components include, but are notlimited to, edge detection, watershed, and connected component labeling.

Referring to FIG. 3, a container 110 is moved on a conveyor 115 througha tunnel 180 in x-direction, perpendicular to the plane of the Figure. Afirst X-ray generator 170, C1, with an X-ray emitting focus projects afan of X-rays 300 through a slit collimator onto an array of detectorsmounted on printed circuit boards 200. One of ordinary skill in the artwould appreciate that only a small sampling of detectors are shown inFIG. 3 and that a typical system would have a far greater number ofdetectors, preferably 700 to 800, more preferably 740. As shown, theorientation of the fan plane is perpendicular to the conveyor surface.While a container is being moved along the conveyor surface, thedetectors are read out repeatedly, and their signals are converted intodigital format by detector electronics that are also mounted on thedetector boards 200. The data are being processed and sorted further andstored in a computer [not shown] for display on a monitor [not shown].Each horizontal line on the monitor corresponds to one particulardetector in the array. Therefore, in a system using 740 detectors, thefull image is composed of 740 lines.

A second X-ray camera, C2, consisting of X-ray generator 171, slitcollimator (not shown) and detector array 201 is mounted in a differentorientation, and offset in conveyor direction, by typically 100 mm. Thedetectors aligned with this camera are sampled essentiallysimultaneously with the detectors of the first camera and produce asecond image displayed on a monitor.

Operationally, an item 340 located within the container 110 isrecognized in the course of the first stage scan using a detectionalgorithm or by operator analysis, depending upon the system modechosen. With the item 340 identified, the approximate centerline X-rayprojections 330, 331 that pass through the object can be determined.Each of the centerlines 330, 331 is associated with a certain detectorchannel, 310 and 311 respectively in each view.

Referring to FIG. 3 a, once the detector channels have been determined,the location of the associated item 340 can be found in the y-zcoordinate system. Two images 380, 381 corresponding to the two viewsare shown. With knowledge of the detectors associated with thecenterlines 331, 330 and the range of detectors, 308 to 314, defined,the y and z coordinates of the item 340 can be derived. The x-coordinateis defined by the direction of conveyor motion and is known because theconveyor motion control system, timing of X-ray exposure, and the fixedoffset of the two scan planes are known. The x-coordinate can, forexample, be referenced to the beginning, or leading edge of thecontainer, which can be detected by a light curtain or similarposition-detecting device. In particular, the two images are referencedto each other precisely in the x-coordinate direction.

The purpose of this triangulation or localization of identified items ina container is to generate control commands that can be used to positionand focus the inspection region or inspection volume of the second stagescanning system on the identified item. Therefore, the first inspectionstage quickly locates potential threats and determines theircoordinates, as referenced to the system, while the second stage focuseson better determining the nature of the identified potential threat. Itshould be appreciated that, because the first stage characterization ofa threat is loosely based on features in X-ray images, it will locate,find, and label, as a potential threat, items which are innocuous, inaddition to real threats. Therefore, the performance of a detectionsystem based only on the first stage, as described, suffers from a highfalse alarm rate.

One of ordinary skill in the art would also appreciate that otherelements of the first stage scanning system are not depicted in FIG. 1but would be included in an implementation of the system. For example, ashielding curtain is positioned at both the entrance and exit of thesystem 100 to protect against radiation leakage to the surroundingenvironment. The system 100 is controlled by a data interface system andcomputer system that is capable of rapid, high data rate processing, isin data communication with storage media for the storage of scan dataand retrieval of reference libraries, and outputs to a monitor having agraphics card capable of presenting images.

It should also be appreciated that a second stage scan may not berequired. In one embodiment, radiographic images from the first stagescan are displayed on a computer monitor for visual inspection withtarget regions or potential threats identified. An operator may dismisssome of the identified regions or threats based on context, observation,or other analytical tools. If no threats are identified, the containeris cleared to exit the inspection system without subjecting it to thesecond stage of scanning. However, if the operator is unable to resolvean area as being a non-threat, the area is identified as a targetregion.

b. The Second Stage

The second stage inspection or scanning system closely inspects theidentified target locations by deriving more specific information, or asignature, and confirming the first stage threat alarm only if theobtained signature matches the signature of a threat substance or threatitem. An alarm confirmed by the second stage system are then takenseriously by operators and indicate the need for further inspection,including, but not limited to, operator image interpretation, additionalscanning, and/or hand searching the container.

In a preferred embodiment, the second stage scanning system usesdiffracted or scattered radiation to determine the properties of amaterial, obtain a signature, and, accordingly, identify a threat.Diffracted or scattered radiation comprises photons that haveexperienced an interaction with the object under investigation. In thespecial case of small angle scattering, the majority of interactions areelastic or energy-conserving; specifically, the diffracted photon hasthe same energy as it had before the interaction, just its direction ofpropagation has changed. If the energy distribution of the scatteredphotons is being analyzed by an energy-dispersive detector system, whichis commercially available, certain properties of the material causingthe scatter are being encoded in the signature. Photons scattered undersmall angles are scattered selectively due to interference effects.Since the process does not change the energy of the photons the signalalso contains the distribution of the primary radiation in a simplymultiplicative way. The incoming primary radiation, as well as thescattered radiation, encounter further spectral modifications due toother types of interactions, such as Compton scatter and photoelectricabsorption, which are not energy preserving. If one wants to view thecharacteristics of the scattering material, other distracting spectraleffects have to be removed.

The detected signature of a threat is therefore a combination of X-rayproperties. One important property is a BRAGG diffraction spectrum,observed at small diffraction angles between 2 and 8 degrees, with apreferred value around 3 degrees.

FIG. 4 shows schematically a cross section of a preferred beam deliverysystem used to obtain BRAGG spectra at small angles. Other beam deliverysystems can also be used in the present invention, including thosedisclosed by Kratky, et al. in Austrian Patent No. 2003753 and Hardingin U.S. Pat. No. 5,265,144. The preferred system depicted in FIG. 4further includes a transmission detector.

A beam delivery system separates the photon radiation emitted by thefocus 400 of the X-ray source 404 into a plurality of beams. A beam 401is formed by passing through apertures 410 and is directly detected bydetectors 402, which are within the beam's direct line-of sight. Thesebeams are referred to as transmission beams. Scatter interactions aredetected by blocking direct line-of-sight detection through the use ofring apertures 410, 411 and exposing the associated detector 420 only toscattered radiation 492. Therefore, scatter radiation, generated whencertain beams interact with an inspection region or volume 445, can bedetected in the same apparatus as transmission radiation.

The choice of ring aperture diameters, distance to focus, and distanceto detector determines the effective scatter angle 430 of the photonsfalling on the detector. In one embodiment, the scatter angle 430 isapproximately the same for substantially all photons detected by thedetector of the scattered radiation. It is preferred to configure thebeam delivery system to establish an effective scatter angle of betweentwo and 8 degrees. It is more preferable to have a scatter angle at orabout 3 degrees. Using a beam delivery system having a circular symmetryhas the advantage of obtaining a scatter contribution from a largervolume of the material being inspected, thereby increasing theinherently weak scatter signal. Additionally, the scatter spectrum canbe cost efficiently detected using only a single detector channel 420with an entrance aperture in the shape of a hole 421.

The scatter signal is generated by positioning the target region 445,identified in the first stage scan, between the beam forming apertures,irradiating that region 445 using the conical beam 442, and making surescatter radiation from the target region 445 can be detected by thescatter detector. The target region 445, often contained within acontainer 450, is in the shape of a tube or ring 445 and is referred toas the inspection volume or inspection region. The length, diameter, andwall thickness of the inspection volume depends on the particular shapeof the elements of the beam delivery system, including focus size, ringaperture diameter and width, detector opening and overall distance. In apreferred embodiment for the inspection of large luggage, the inspectionvolume is at or about 60 cubic centimeters.

In a preferred embodiment, as shown in FIG. 5, the components of thebeam delivery system are mounted to the open ends of a rigid supportstructure 500 formed in the shape of a C (referred to herein as a C-arm)and aligned with a tolerance of at or about 0.1 millimeters. A first armof the C-arm comprises a X-ray tube with X-ray focus 172, a beamlimiting aperture hole mounted to the tube head 401, and a ring-shapedaperture 410. A second arm holds comprises a transmission detector array402, a second ring aperture 411, and an energy dispersive detector 420,equipped with an aperture hole.

The energy dispersive detector 420 is positioned to receive scatteredradiation from a target object placed on the conveyor running betweenthe arms of the C-arm support structure where a first arm is above theconveyor and a second arm is below the conveyor. The transmissiondetector is positioned to receive radiation attenuated by the sametarget object. It is preferable for the C-arm to be mobile and capableof moving in the x-direction along the length of the conveyor.Therefore, the C-arm with tube and detectors can be re-positioned alongthe length of the conveyor.

In a preferred embodiment, the scatter detector 420 is comprised ofcadmium telluride or cadmium zinc telluride and is operated at roomtemperature, or approximate to room temperature, An exemplary embodimentis available from the e-V Products Company, Saxonburg, Pa. This type ofdetector has a spectral resolution performance that is well matched tothe limited angular requirements of this application, and therefore thelimited spectral resolution of the beam delivery system.

In one mode of operation, the potential threat locations inside acontainer are found automatically by the first stage, and, based uponthe physical coordinates obtained through triangulation, the secondstage scanning system is automatically positioned to generate aninspection region that substantially overlaps with the identified targetregion. Where multiple threat locations are identified, the second stagescanning system is sequentially repositioned to focus on each subsequenttarget region. To scan each target region, the second stage X-ray sourceis activated and the scatter detector and transmission detector aresampled simultaneously. In a preferred embodiment, a transmissionspectrum associated with the detected transmission data is characterizedusing a look up reference, figure, table, or chart, and the scatterspectrum is normalized using that identified transmission spectrum.

In another mode of operation, an operator actively identifies imagesthat he or she believes corresponds to a potential threat. X-ray imagesfrom the first inspection stage are displayed to the operator, and theoperator points to a suspicious object as it appears in both views. Tosupport this functionality, operators use a computer system, comprisinga mouse and monitor, to position cross hairs over the areas of intereston each of the images. Using coordinate data generated throughtriangulation, the second stage scanning system automatically positionsitself such that an inspection region overlaps with the target region,activates the X-ray source and simultaneously samples the scatterdetector and transmission detector. In a preferred embodiment, atransmission spectrum associated with the detected transmission data ischaracterized using a look up reference, figure, table, or chart, andthe scatter spectrum is normalized using that identified transmissionspectrum.

c. The Transmission Detectors

As discussed above, a transmission detector is integrally formed withthe beam delivery system, as shown in FIGS. 4 and 5. A preferredtransmission detector comprises a 16 channel array of dual energydetectors. The detector array further comprises pairs of detectors,including a low energy channel that receives and measures a first amountof radiation first (low energy) and a high energy channel that receivesand measures a substantial portion of the balance of radiation (highenergy). Dual energy detection has been described in connection with thelinear scan arrays of the first inspection stage and is known to personsof ordinary skill in the art.

The low energy and high energy detectors measure a plurality of lowenergy and high energy values that can be used to characterize thematerial being scanned. In a preferred embodiment, low energy and highenergy data are used to reference a look up reference, figure, table, orchart (referred to as a look up source) which contains transmissionspectra arranged in accordance with corresponding high and low energyvalues. The look up source is constructed with high energy values on oneaxis (i.e. the x-axis), and low energy values on a second axis (i.e. they-axis). Referring to FIG. 6, an exemplary look up source 600 is shown.The source 600 is a graph with high energy values on the x-axis 605 andlow energy values on the y-axis 610. Points 615 corresponding tomeasured spectra 620 are positioned on the graph according to certainlinear combinations of the measured high and low dual energy detectorsignals on the x and y axis.

The transmission spectra used to normalize scatter data is thereforeidentified by obtaining high energy and low energy data values,identifying the point on the graph corresponding to the detected highand low energy values, and looking up the spectrum associated with thatpoint. Where the detected high and low energy values yield a point on agraph that corresponds to an intermediate point 630 proximate topre-established points 635, 615, a corresponding transmission spectra645 can be calculated by performing a two-dimensional interpolation ofthe spectra 640, 620 associated with the pre-established points 635,615.

To create the look up source, an exemplary approach places variousmaterials of known composition and thickness, exposes them to X-raysources, measures the resulting high and low energy data values, anduses the scatter detector to measure the corresponding transmissionspectrum. More specifically, the beam path of the beam delivery systemis modified to allow a direct beam from the focus through the pinhole tofall on the energy dispersive scatter detector. To further reduce thephoton flux into a range that can be tolerated for energy-dispersivemeasurement, the current of the X-ray source is preferably reduced by alarge factor, e.g. 100. Under these parameters, the scatter detector canbe used to measure the transmission spectrum. Materials of knowncomposition and thickness are placed in the beam path. The materials areexposed to X-ray radiation. Dual energy measurements are made using thedual energy detectors and a transmission spectrum is obtained using thescatter detector. Through this approach, for each material compositionand thickness, a transmission spectrum is obtained and correlated withdiscrete pairs of dual energy transmission detector readings. Thisinformation is then arranged on a chart with the high energy value ofthe dual energy detector measurement on the x-axis, and the low energyvalue on the y-axis.

It should be appreciated that, in the disclosed embodiment, the spectraare the looked-up objects of the look up source. Instead of the spectra,however, the look up source can alternatively consist of spectralattenuation functions related to the attenuation of the materials placedin the beam when the look up source is being generated. The spectrum canthen be obtained by multiplying one fixed spectrum, for example thespectrum measured without the material placed into the beam, with thespectral attenuation function retrieved from the look up source.Alternatively, the look-up source can contain numbers that are theparameters of analytical expressions, e.g. polynomials, which are formedto describe the attenuation functions in a parametric way.

The presently described approach is preferred because it enables theconstruction of a transmission detector array from lower cost materials,as opposed to constructing the array using more expensive energydispersive detectors and support electronics. Moreover, it alsoaddresses the difficult problem of using energy dispersive detectors tomeasure transmission spectra at the high flux rates that are experiencedat the location of the transmission detector in the given configurationand at the same time at which the scatter data are recorded. Therequired strong attenuation of the transmission beams is a difficultproblem that is avoided using the present invention. The look up tableis an important element because the preferred dual energy detectors usedin the transmission detector cannot deliver spectra directly.

As discussed, transmission spectra are being used to correct the scatterspectra that are being recorded by the energy dispersive detector.Normalizing scatter spectra with transmission spectra corrects for theconfounding effects introduced by the specific spectral distribution ofthe primary radiation, as emitted from the X-ray source, as well as byspectrum-distorting effects known as beam hardening. To correct thescatter spectra, the detected scatter spectra are divided by thelooked-up transmission spectra.

A normalized scatter spectrum exhibits a plurality of features. A firstfeature is that the location of the peaks and valleys of the spectrumare determined by the molecular structure of the materials located inthe probe region. A second unrelated feature is that the averagespectral signal of the normalized scatter signal, which can be ofvarying intensity, is linearly related to the gravimetric density of thematerial in the probe region. This can be used for threat discriminationsince most explosives, particularly military explosives, have a densityrange above that of most other plastic or food items in suitcases.

In one embodiment, the normalized scatter signal is used to identify athreat item by comparing the obtained normalized scatter spectrum and/orspectral signal with a library of scatter signals from known threatitems. This comparison can occur automatically by using a processor tocompare a library of threat items, stored in a memory, with the obtainedscatter signals. Such a library is developed by measuring the normalizedscatter signatures of known threat items. In addition to using thetransmission detector to generate data used to identify referencespectra, the transmission detector can function in a plurality of otherways. In one embodiment, the transmission detector acts as a positionsensor. The transmission beam is interrupted or attenuated momentarilywhen an object on the conveyor crosses it. Tracking the moment ofinterruption can provide information on the physical position of thecontainer on the conveyor and be used to appropriately position the beamdelivery system or container.

In a second embodiment, the transmission detector array functions as animaging detector to provide precise attenuation data for certain areasin containers, like container wall areas, where contraband can behidden. When the circular beam is centered on an edge of a container,the edge of the container can be imaged in good detail, and can helpanalyze the edges for concealed threats.

In a third embodiment, transmission detector measurements can be used todetermine whether the inspection region is, in fact, the same targetregion previously identified in the first stage scan. If thetransmission data correlates with X-ray characteristics different thanthose obtained in the first stage scan, the relative positioning of thesecond stage scanning system and the object under inspection may bemodified until the transmission data correlates with the same materialcharacteristics that was identified in the first stage scan.

In a fourth embodiment, transmission detector data are also being usedto simplify the algorithm-training procedure of the system, as describedbelow, in particular the collection of threat material properties withirregularly shaped threat samples, like sticks of dynamite.

It should be noted that it would appear because the scatter radiationpath and transmission path differ downstream from the scatter volume,there would be inconsistencies in the data when scatter and transmissiondata are combined. This inconsistency is one example of a number ofpartial volume effects, solutions for which are addressed herein.However, the inconsistencies are not significant and can be toleratedwithout encountering significant performance degradation of the systemas a whole. As shown, FIG. 4 is not an isometric schematic and, inreality, the scatter angle is preferably about 3 degrees, and the realpath differences are comparatively smaller.

d. Positioning Inspection Regions

As previously discussed, the second stage scanning system positions aninspection region to physically coincide with the target regionidentified in the first stage scan. The positioning means may beachieved using any method known in the art. In one embodiment, aplurality of control commands is produced in response to thedetermination of the location of the target region. The control commandsare generated by at least one processor in data communication with aplurality of processors capable of executing the aforementionedtriangulation techniques and/or determining the intersection ofprojection lines to identify the location of the target region in threedimensional system coordinates.

The control commands comprise data signals that drive a three-axiscontrol system. The vertical position of the second-stage inspectionvolume can be adjusted to the target volume or region of the first stagescan by moving the conveyor system up or down. In another embodiment,the control commands comprise data signals that drive the adjustment ofthe beam delivery system in the second stage scanning system. The beamdelivery system adjustment can include any type of adjustment to thecollimation or beam focus, including the physical movement of aplurality of apertures horizontally, vertically, or diagonally, thephysical modification of the diameter of the ring aperture by, forexample, increasing or decreasing the aperture size. In anotherembodiment, the position of the support structure, or C-arm, can bemodified along the conveyor direction to appropriately position the beamdelivery system.

The second stage scan may be compromised when the volume of the targetregion is smaller than the inspection region of the second stage. Insuch cases, extraneous material, other than the material identified asbeing a potential threat, such as air, metal, or container edges, may beincluded. The resulting scatter radiation is therefore a function ofmultiple material types and may not be readily identifiable as being thesignature of a single substance.

e. Threat Recognition Process of the Preferred Embodiment

In one embodiment, the present invention comprises a threat recognitionprocess that incorporates a training methodology which relies onlibraries in which threat signatures are obtained by combining thethreat with other common materials, such as clothing, plastic, air, andmetals. Specifically, the data used in training and developing thedetection process are chosen to include data, which are corrupted byerrors based on partial volume data from statistically varyingcontainers and threat and non-threat material combinations. When theinspection volume is partially filled with a threat substance andpartially filled with a second innocuous substance, a combination signalwill be detected by the second scanning stage. The automatic threatrecognition methodology recognizes the threat from the combinationsignal based upon the aforementioned training. An exemplary automaticthreat recognition methodology, based on neural networks, is describedbelow.

In a second embodiment, the detected scatter data is corrected for theeffects of extraneous materials by preprocessing the data. The motioncontrol system tracks where the inspection volume or region is locatedin relative to a specific reference point, such as the approximateoutlines of the container, and relative to the conveyor system. Becauseof the ability to measure and track these reference points, the amountand portion of the inspection volume occupied by the conveyor structurecan be determined. The conveyor structure includes the belt material aswell as the structural member that is underneath the conveyor, which isreferred to as the slider bed.

To correct the scatter spectrum for the presence of the conveyor in theinspection volume, the scatter spectrum of the conveyor materials ismeasured and stored in a reference database. When the scatter spectrumof the inspection region is detected and it is determined that theconveyor occupied a portion of the inspection region, the scatterspectrum is corrected by multiplying the conveyor material scatterspectrum by a weighting factor to account for the size of the inspectionvolume occupied and that amount is subtracted from the measurement.

Similarly, when part of the inspection volume is filled with air, as incases when suitcase walls are targeted by the inspection volume, it isknown that the contribution of the air-filled portion of the inspectionvolume to the scatter signal is approximately zero, and therefore,substantially all of the scatter signal can be attributed to thematerial in the remainder of the inspection volume. By accounting forthe air volume contribution, the characterization of the material in theremaining inspection volume is rendered more precise. Optical detectors,such as a plurality of light-curtains, can be positioned across andwithin the scanning system to generate control signals that conveyinformation about the height and edges of the container relative to theconveyor system and relative to the inspection region. It therefore canbe calculated which portion of the inspection region is filled with air.

In another embodiment, transmission values for the scatter beam aremeasured by an array detector. An exemplary array comprises 16 channelsand yields transmission data for 16 subdivisions within the inspectionvolume. The transmission values can be used to characterize the materialdistribution in the inspection volume. Based on these transmissionvalues, approximate mass values can be determined for masses containedin each of the 16 subdivisions. For example, where the transmissiondetector value returns a value indicating the subdivision has materialwith zero thickness, it can be assumed that the subdivision is occupiedby air.

In a preferred embodiment, the inspection volume is subdivided. Byreducing the size of the inspection region, one can ensure that fewerdiffering materials occupy the same region and can therefore avoid thecomplex composite signals that get generated when multiple materialsfill a single inspection region. In one embodiment, system resolution isincreased by providing multiple energy dispersive detectors, such as 2,3, 4, 5, 6 or more, in place of a single energy dispersive detector asshown in FIG. 4.

Referring to FIG. 7, a schematic representation of the beam deliverysystem of FIG. 4 780 is shown relative to a beam delivery system havingmultiple energy dispersive detectors 785. A first system 780 comprisessingle detector 700 s, circular aperture 701 s, inspection volume 702 s,circular aperture 703 s, and X-ray focus 704 s. The dark areas representthe presence of radiation blocking material, e.g. ¼ inch lead alloy, andthe white areas represent areas that are transparent to X-rays abovekeV. A second system 785 comprises an X-ray focus 704 q, circularaperture 703 q, divided inspection volume 702 q, detector side beamshaping aperture 701 q, and quadruple detector 700 q. The aperture 701 qis center-symmetric and consists of four slits, each conforming to partof a circle. The centers of the circular slits are chosen to be of thesame pattern as the detectors of the quadruple detector 700 q. Forexample, if the detector cluster consists of four channels centered onthe four corners of a 2 by 2 mm square, the centers of the partial andcircular apertures lay on a circle with diameter equal to the squareroot of 2 times 2 mm. The resulting inspection region for eachindividual detection region is about one quarter of the full inspectionvolume. A subdivided inspection region provides a higher spatialresolution of the second stage inspection. Clusters of energy dispersivedetectors with their supporting electronics are commercially availablefrom companies such as eV Products, Saxonburg, Pa.

If more than one scatter detector is being employed, a collimatingsystem of vanes can be placed in front of the detector clusterorthogonal to the surface of the detector and in line with the plane ofseparation between each detector. Using a separator 705, diffractedradiation is more effectively limited to reach the appropriate channelin the cluster and, consequently, detected signals are more readilyassociated with materials from specific areas within the inspectionregion. The separator 705 extends from the surface of the detectorcluster toward the surface of the adjacent aperture. The number ofseparator vanes is dependent on the number of detectors. A typical vanematerial and thickness is lead alloy of 0.5 mm thickness.

f. Operational Process of the Preferred Embodiment

Referring to FIG. 9, a flowchart summarizing the operational process ofone embodiment of the present invention is provided. A container entersinto the first stage scan 905 where it is exposed to a plurality ofprojected beams 915. From that exposure, X-ray characteristics aredetermined 920 and target regions containing potential threats areidentified 925, 935. If no potential threats are identified, thecontainer is not subjected to a second scanning stage 940. The threedimensional coordinates of the target region is determined 945 and,accordingly, the inspection region generated by the second stagescanning system is coordinated to coincide with the target region 950.The inspection region is subjected to X-ray radiation in order to obtaintransmission and spectral data 955. The spectral data is then analyzed960 to determine the existence of a threat. The data collected in thesecond stage scan comprises both localized dual energy transmission dataand localized BRAGG diffraction spectra, which are subject tostatistical variances, originating from photon signal fluctuations,partial volume limitations, or variations of the type of luggage andtheir contents, among other causes. As such, it is preferred to have aprocessing methodology that accounts for the fact that the raw data isnot sufficiently sensitive to detect threats with sufficiently low falsealarm rate.

In a preferred embodiment, the automatic threat resolution is performedby a probabilistic technique in which a plurality of input data points,obtained from the raw spectral scan data, contribute to the probabilitythat the corresponding spectrum belongs to a particular class of threator non-threat items. Such a probabilistic technique relies on theplurality of input data points as a whole rather than on individual datapoints. Although probabilistic classification techniques can includeexplicit, identifiable rules created by a programmer, the preferredtechniques utilize a classification procedure that incorporates theresults of training. For example, the classification algorithm can beused to process a training set consisting of patterns for structures ofknown classification. The results of this processing are used to adjustthe algorithm, so that the classification accuracy improves as thealgorithm learns by processing the training sets.

One type of trainable classifier that can be employed is an artificialneural network. Artificial neural networks attempt to model humanbiological neural networks to perform pattern recognition and dataclassification tasks. Neural networks are fine grain parallel processingarchitectures composed of non-linear processing units, known as neuronsor nodes, which attempt to replicate the synaptic-dendriticinterconnections found in the human brain.

Different types of neural networks exist. One type of network is amulti-player, feed-forward network. A feed forward network passes asignal by links from input nodes to output nodes, in one direction only.In most implementations, the nodes are organized into multiple layers:the input layer, output layer, and several “hidden layers” in between.The adjacent layers are normally fully interconnected.

FIG. 8 depicts a schematic representation of a preferred type ofartificial neural network 800 known as a hidden-layer feed-forwardnetwork consisting of an input layer 810 of neurons or nodes, at leastone hidden layer 820, and an output layer 830. The neuron layers arelinked via a set of synaptic interconnections. Each neuron in the inputlayer is typically connected to each neuron in the hidden layer, andeach neuron in the hidden layer is typically connected to each neuron inthe output layer, via a synaptic connection; these may be physical,electronic connections, or they may be embodied in software, as may bethe neurons themselves, which software operates on conventional digitalcomputers.

The neurons or nodes typically accept several inputs and create aweighted sum (a vector dot product). This sum is then tested against anactivation rule (typically a threshold) and then processed through anoutput function. The output function could be a non-linear function suchas a hard-limiter; a sigmoid function; a sine-function or any othersuitable function known to a person of ordinary skill in the art. Thethreshold determines how high the input to that neuron must be in orderto generate a positive output of that neuron. A neuron may be consideredto be turned on, for instance, whenever its value is above apredetermined value such as, for instance, 0.9 and turned off with avalue of less than another value such as 0.1, and has an undefined“maybe” state between those values. The connectivity pattern defineswhich node receives the output value of a previous node as their input.The connection between two neurons is realized in mathematical terms bymultiplying the output of the lower level neuron by the strength of thatconnection (weight). At each instant of propagation, the values for theinputs define an activity state. The initial activity state is definedupon presentation of the inputs to the network.

The output response of any hidden layer neuron (o_(j)) and any outputlayer neuron is a function of the network input to that neuron definedby the difference of that neuron's threshold (θ) and the input to it.The value of the input into each hidden or output layer neuron isweighted with the weight currently stored for the connection strengthsbetween each of the input and hidden layer neurons, and the hidden andoutput layer neurons, respectively. Summing over all connections into aparticular neuron and subtracting this sum from the threshold value maybe performed according to the following sigmoid-type Fermi function:

o _(j)=[1+exp(θ_(j)−Σ_(i) ·w _(ji) *o _(i))]⁻¹

where i and j represent neurons of two different layers with jrepresenting the higher layer; θ_(j) represents the bias value for jlayer neuron; and w_(ji) represents the strength of the connectionbetween neuron i and neuron j. Alternatively, sine-type functions, orany other suitable function known in the art, may be used to obtain thedesired type of response function for the output of a neuron. Theweights are chosen so as to minimize the error between the producedresult and the correct result. A learning rule defines how to choose theweight values. Several commonly used learning rules areback-propagation, competitive learning, adaptive resonance, andself-organization.

In a preferred embodiment, the artificial neural network usesback-propagation learning. The back-propagation learning algorithm,derived from the chain rule for partial derivatives, provides a gradientdescent learning method in the space of weights and can be furtherunderstood by reference to D. E. Rumelhart, et al., Parallel DistributedProcessing, ch. 8, pp. 322-28 (MIT Press, 1986) and Haykin, Simon(1999), “Neural Networks”, Prentice Hall, both of which are incorporatedherein by reference.

Back-propagation learning involves a set of pairs of input and outputvectors. The network uses an input vector to generate its own, oractual, output vector. The actual output vector is compared with adesired output, or target, vector that may be defined usually in thecourse of training. The weights are changed to obtain a match betweenthe target vector and the actual output vector. The conventional deltarule may be used for this calculation where the weight for a particularsynapse or connection between units is adjusted proportionally to theproduct of an error signal, delta, available to the unit receiving inputvia the connection and the output of the unit sending a signal via theconnection. If a unit is an output unit, the error signal isproportional to the difference between the actual and target value ofthe unit. If it is a hidden layer, it is determined recursively in termsof the error signals of the units to which it directly connects and theweights of those connections.

Thus, the training of a neural network is the process of setting theconnection weights so that the network produces a desired output inresponse to any input that is normal for the situation. A supervisedtraining refers to the kind of training that requires a training set,i.e. a set of input-output patterns. The back-propagation algorithm isan efficient technique to train a feed-forward network. It operates tosend an error back through the network during the training process,thereby adjusting all the link weights in correspondence with theircontribution to the error. The weights of the network thereforegradually drift to a better set of values. The initial weights arechosen randomly within reasonable limits and adjustments are left to thetraining process.

Referring back to FIG. 8, the artificial neural network 800 is trainedon a suitably large set of threat and non-threat X-ray raw scan data, togenerate an output 840, in accordance with the error back-propagationlearning method described above. As described earlier, the required setof threat and non-threat raw scan data for training can be obtainedeither from the scanning system of the first stage or the scanningsystem of the second stage or both depending upon whether artificialneural networks are used to process scan data from the first stage orthe second stage or from both the stages. Thus, the ‘scan data’ 805 tobe used to train the neural net 800 may comprise of raw attenuationdata, raw transmission photon counts, raw diffraction photon spectra orany other data known to a person of ordinary skill in the art.

The purpose of the neural network processing step is to have aprocessing means capable of recognizing a threat signature. A threatsignature is defined as a spectrum, i.e. an array of numberscorresponding, on a one-to-one basis, to the discretized values of aphysical quantity, such as the energy of X-rays, and includes unrelated,but relevant, other values, such as transmission detector array data,bag height, and other environmental factors. Although the spectrum mayconsist of any amount of data points, the present invention preferablyoperates on a spectrum data set of between 200 and 800 points and, morepreferably, of approximately 500 points. Additionally, while the networkmay consist of any number of layers, it is preferred that it consists offour layers, including one input layer, two hidden layers, and oneoutput layer. Further, while the network can have multiple output nodeswith various indicators, it is preferred that, for the presentinvention, the network comprise a single node the output of which may beinterpreted as either “yes, a threat has been recognized” or “no, athreat has not been recognized”.

In a preferred embodiment, the raw diffraction spectrum data from thesecond stage is used to generate the required set of threat andnon-threat scan data for training. These spectral counts represent raw,that is non-normalized, scan data 805 that is subsequently used to trainthe neural network 800. Alternatively, this scan data 805 may be furtherprocessed to generate a plurality of normalized data.

The scanning process is repeated to obtain scan data of a sufficientlylarge number of containers containing threat and non-threat itemspackaged in a variety of permutations and combinations to modelreal-world scenarios. This raw scan data, referred to hereinafter astraining data, comprise an input-set to be used for training the neuralnetwork. Since the training data is obtained by scanning containerscontaining known materials, each output training data maybe furthertagged to identify whether the respective training data represents adefined/known threat or non-threat item. This output training data maybefurther stored in a suitable library or database such as a file serveron a digital computer system along with the tagged identificationinformation. Furthermore, the library or database of training data maybe enhanced to incorporate and reflect all previously known threatmaterials and their corresponding raw ‘scan data’.

In a preferred embodiment, two libraries are generated. A first libraryhas signatures of threats. A larger second library has signatures ofinnocuous, or non-threat, items. The training process utilizes thethreat and non-threat signatures to introduce into the systemthreat-like and non-threat-like signatures.

A threat-like signature is a linear combination of a sample from thethreat library with a plurality of samples, such as two, from thenon-threat library. The coefficients of the mix are randomly simulated.A simulated white noise is also added to the generated mixture, with itsamplitude also randomly generated, within an interval from zero to agiven fraction of the signal. A non-threat-like signature is a mixtureof a plurality of non-threat signatures, such as two or three. Thecoefficients of the mix are randomly simulated. A simulated white noiseis also added to the generated mixture, with its amplitude also randomlygenerated, within an interval from zero to a given fraction of thesignal. Using signature mixes, incorporated with noise, the system istrained to recognize a threat or a non-threat by outputting anappropriate recognition answer from the last output node within thecontext of a reasonable level of noise and interference from overlappingitems.

FIG. 10 depicts a plurality of steps, in flow diagram format, of oneembodiment of the back-propagation training process of the invention.One of ordinary skill in the art would appreciate that the processing isconducted using a computer having a plurality of processors forexecuting the analytical processes described herein, embodied in atleast one software program, a plurality of storage devices for storingthe requisite data, library information, and other information necessaryto conduct these analyses, and an output device, such as monitor, amongother commonly known computing devices and peripherals.

At the beginning of the training process 1003, the synaptic weights andthresholds of the neural net are initialized 1005 with, for example,random numbers. After initialization 1005, the input layer of the neuralnetwork is introduced 1010 to a first set of training data and the netis run to receive 1015 an actual output. The neural net makes use of therandomly assigned weights and thresholds to generate an output on thebasis of a suitable resolving function such as a sigmoid-type Fermiequation (described earlier), a sine function or any other functionknown to a person of ordinary skill in the art. The output could be inthe form of differentiable signals such as numerals between, say, 0 and1, in the form of positive or negative states implied by an outputnumeral of greater than or less than 0 respectively, or any othersuitable indication as evident to a person of ordinary skill in the art.

The first set of training data is introduced into the system and, basedon the random weights and thresholds, produces an output ‘x’, i.e. anumeral greater than 0. If the training data represents a threat, thisoutput indication is set as a benchmark to identify a ‘threat’ while anumeral less than 0 maybe set to identify a ‘non-threat’ item. Once asuitable benchmark is set, the training process is repeated with thenext set of training data and corresponding actual outputs are received.The actual output is compared 1020 with the desired output, defined byan operator with knowledge as to whether input data is or is notrepresentative of a threat, for the corresponding set of training datathat was fed to the neural net in step 1010. If the actual outputreceived 1015 is commensurate with the desired or targeted output or, ifthe difference between the target and actual output falls below apredefined acceptable level, a check 1025 is made to see if the neuralnet has been trained on the entire set of training data. If not then thenext set of training data is introduced to the neural net and theforegoing steps are repeated. The training process continues until theneural net has been trained on the entire set of training data.

If the comparison 1020 suggests that the actual output is not inagreement with the desired or targeted output, the ensuing additionalsteps are performed. The difference between the actual and desiredoutputs is used to generate 1030 an error pattern in accordance with asuitable back-propagation rule such as the ‘delta rule’ or any othererror estimation rule known to a person of ordinary skill in the art.The error pattern is used to adjust 1035 the synaptic weights on theoutput layer such that the error pattern would be reduced the next timeif the same set of training data were presented as the inputs. Then theweights of the hidden layers, preceding the output layer, are modified1040 by comparing what outputs they actually produce with the results ofneurons/nodes in the output layer to form an error pattern for thehidden layer.

The error can thus be propagated as far back over as many hidden layersas constituting the neural network. Finally, the weights for the inputlayer are similarly educated 1045, and the next set of training data isintroduced to the neural network to iterate through the learning cycleagain. The neural network is therefore trained by presenting each set oftraining data in turn at the inputs and propagating forwards andbackwards, followed by the next input data, and repeating this cycle asufficient number of times such that the neural network keeps gettingcloser and closer to the required weight values each time. Thus, thenetwork, through the iterative back-propagation process, establishes aset of weights and thresholds for neural connections so that a desiredoutput pattern is produced for the presented input information. Thelearned information of a neural network is contained in the values ofthe set of weights and thresholds.

In exemplary embodiments, the neural network is structured such that,through iterative forward and backward propagation, every node in alayer can be made to contribute to every node in a subsequent layer,only certain nodes in a layer maybe used to contribute to certain nodesin a subsequent layer, or every node in a layer contributes to everynode in a subsequent layer but the impact of certain first layer nodeson subsequent layers are weighted relative to other first layer nodes.In a preferred embodiment, the nodes closest to subsequent layer nodesare weighted relative to other nodes in that same layer.

More preferably, links between the input layer and the first hiddenlayer are not chosen randomly, but selected to have a specialdistribution. Each hidden layer node is responsible for a region of thespectrum. Links to each hidden layer node from this region have higherweights. Therefore, the farther an input node is from this region, andthe less responsible it is, the weaker the link with that input node.Together, the hidden nodes encompass the entire input layer spectrum. Bydistributing a pre-assigned degree of influence over links, a form ofconvolution is provided. This embodiment is particularly preferred wherepreprocessing is not reasonably effectuated because the input data sizeis too large.

Because new threats may develop over time, it is desirable to have asimple procedure that updates the network to recognize such additionalthreats. In a preferred embodiment, multiple networks are formed andtrained to identify distinct threats. Therefore, new threat recognitionis done by implementing a neural network as a set of multiple networks,each trained to identify a specific threat.

Each network group is formed and trained to address and recognize onethreat. Thus, there is a one-to-one correspondence between the threats(T₁, T₂, T₃ . . . T_(n)) and the groups (G₁, G₂, G₃ . . . G_(n)).Network group G_(n) is trained to recognize threat T_(n). Where G_(n)recognizes any other threat, T_(n−2), T_(n−1), T_(n+1), it is notconsidered relevant. G_(n) is trained using threat signatures of T_(n)and a corresponding library of non-threat signatures. A group consistsof a plurality, such as two, three, or four, completely separatenetworks, each similarly trained to recognize the same threat. Threatrecognition is achieved where the average of the recognition results ofeach network indicates a threat. Where an additional threat isidentified, T_(new), a new group of networks, G_(new), can be created,without having to retrain or modify all existing groups. This permitsthe more efficient, incremental addition of new threat recognitionnetworks.

One of ordinary skill in the art would appreciate that the output ofthese network groups can be handled in various ways. Specifically, asystem can output a threat alarm if the recognition result, averagedover all network groups, indicates a threat. A system can output athreat alarm if only one group of networks indicates a threat or if asubset of network groups indicates a threat. In a preferred embodiment,threat recognition is effectuated by monitoring the output ofsubstantially all groups of networks. If the recognition result,averaged over all groups, indicates the existence of a threat, theoutput results of individual network groups are analyzed. If at leasttwo groups indicate the presence of a threat, then the groups arereviewed to determine which threat has been recognized. One of ordinaryskill in the art would appreciate that various derivations of theabove-described process can be conducted without departing from thescope of this invention. For example, the threshold analysis can beperformed even if only a portion of all network groups is monitored andthe secondary analysis can be performed if fewer than two groupsindicate a threat.

It is further preferred to regulate the balance between the sensitivityof detection with the selectivity of detection by incorporating anadditional input node that is used to inject a sensitivity level intothe training process. Systems with higher sensitivity will detect morethreats at the expense of having greater false alarms. Systems withhigher selectivity will have fewer false alarms, with the disadvantageof possibly not detecting all threats. In the course of operation, itmay be necessary to change the sensitivity/selectivity balance,depending on various circumstances, and therefore it is desirable tohave a means for doing so.

In one embodiment, a plurality of different networks is stored in astorage device in data communication with processors responsible forexecuting neural network analytical processes. Each set of networks hasa different level of sensitivity, i.e. least sensitive, less sensitive,normal sensitivity, more sensitive, most sensitive. Depending on therequisite level of security (versus requisite level of throughput), theappropriate network set is loaded into the system. Alternatively, anetwork set having a standard level of sensitivity may be used inoperation and a parallel network set, having varying levels ofsensitivity, may be concurrently loaded and ready for use, whennecessary. Having a parallel network avoids downtime associated withloading new network sets into local memory, or RAM.

Networks having varying degrees of sensitivity are developed byincorporating a sensitivity variable in the training process. With eachrecognition task, a desired level of sensitivity is communicated to anadditional input layer node, thereby inherently incorporating it intothe training process. Every training event could further be associatedwith a randomly selected sensitivity level, selected from within areasonable range. Training is therefore conducted with the selectedlevel of sensitivity.

The embodiment of back-propagation learning process, as described withreference to the flow diagram of FIG. 10, assumes that the training datais first collected by operating the first and/or second scanning systemsoffline, namely by scanning a large number of baggage containing knownthreat materials camouflaged amongst non-threat items in a variety ofcombinations to represent a variety of concealment scenarios. Thetraining data so obtained is then used to educate the neural networkthat can then be used to operate in real-world situations. Thus, in thisembodiment, when the first and/or second stage systems are online, thatis operational at real-world sites such as at airports for luggageinspection, the associated neural network is already partially educated(on the training data obtained through test baggage) to discern threatfrom non-threat items.

However, in another embodiment of the back-propagation learning process,the neural network need not be taught through the scanning of testbaggage prior to running the first and/or second scanning systemsonline. Instead the first and/or second scanning systems can be operatedonline and the scan data can then be fed into the neural network inreal-time. On the basis of this real-time scan data, the neural net ismade to classify threat and non-threat items. At the same time, thescanned image of the baggage is also presented to an operator in theform of a visual display such as on a conventional computer screen, asis known in the art. The operator compares the output of the neural netwith his own observation of the scanned baggage. In case the output ofthe neural net is found to be erroneous, the operator prompts the neuralnet with the correct or desired output, enabling the neural network toadjust its weights and thresholds accordingly. Thus, in the secondembodiment when the system is first used, it will have relatively littleknowledge about threat and non-threat materials to be identified andrecognized. However, with sufficient positive reinforcement of arelationship between the acquired scan data and operator promptedidentification, the neural network will learn how to identify threatobjects. This self-learning process enables the neural network to learncontinuously.

The on-line, self-learning training process does have certainadvantages. A company or organization that uses the present inventionmay not want to share or disclose data to third parties due to privacyor security reasons. Therefore, it may be essential to enableself-learning. Furthermore, the flow of data may change seasonallydepending upon how containers change. Specifically, seasonal variationsdo occur in airline passenger travel where passenger bags may get largerin the winter to accommodate more clothing or the number of total bagsmay increase due to larger numbers of people traveling in the summer. Toaddress such seasonal variation, it is more practical to allow on-lineautonomous adaptation.

Finally, there may be a variety of system users in differentgeographical regions that experience different types of threat andnon-threat items. In such cases, a standard library may not be ashelpful as self-taught systems that automatically learn in accordancewith its own unique context. More specifically, over time, a system inthe field will be trained on containers that have threat and non-threatitems unique to their geographic context. Due to operator training andinteraction, a particular system would therefore develop a trainedprocessing system tailored to their geographical context. It ispreferred, however, that, irrespective of the geographical location,systems get periodically trained using threats that are new orinfrequently seen to ensure that the system does not forget the identityof such threats. This update could be performed by the statisticallycontrolled re-injection of threats from existing threat databases.

Although this online self-learning process has been described separatelyas an embodiment, this continuous self-learning process can be used inconjunction or combination with the offline teaching process of thefirst embodiment, using test baggage. In a preferred embodiment, theneural network is first trained on scan data obtained by running theX-ray system offline on test baggage and then through operator promptsin real-time operations as well, so that the ability of the neural net,in identifying objects, continuously improves through self-learning.Nevertheless, the system may undergo retraining offline using data frommultiple site locations, thereby taking full advantage of the sum totalof learning being generated by the operation of multiple systems.

Operationally, acquired scan data is fed into the neural network foridentification. If the object is identified with a high degree ofconfidence, the identification and scan image is conveyed to anoperator, along with an indication of what the object may be, to enablethe operator to take an action, including conducting a hand search,questioning the container owner, permitting the container to pass, orcalling in additional personnel. In one embodiment, the operatorprovides feedback to the system based upon the identity of thethreat/non-threat and action taken. For example, if the systemidentifies the existence of a threat, the operator can check thecontainer to determine if a threat exists and then inform the systemwhether it was or was not correct. If correct, the neural networkincreases its confidence factor for that object's scan data and storesthe scan data in a suitable database as an exemplar for retraining. Ifincorrect, the neural network implements the error back-propagationprocess to suitably adjust its weights and thresholds and stores thescan data in a suitable database as an exemplar for retraining.

This on-line adaptation process using incoming data requires certainprecautions, however. It is preferred that the system utilizes groups ofnetworks, which are accompanied by libraries of threat patterns andinnocuous patterns, and that the system is not authorized to modifythese libraries. It is further preferred to include an additionallibrary, a buffer-library, that is available for modification based uponincoming data. Specifically, the buffer library comprises incoming newdata, and is preferably periodically cleansed of older data.Consequently, the networks are being re-trained using the buffer-libraryand the two stable libraries, with a proper adaptation time scale.Several previous versions of network groups are stored as a back up anda comparison of newly adapted system with its older versions can beconducted and produced in the form of a report. As described, on-sitetraining can be set up as an automatic feature, but operator input maybe required in the rare case of a real alarm.

IV. Alternate First Stage and/or Second Stage Systems

a. Nuclear Quadrupole Resonance (NQR) Employed as a First or SecondStage Scanning System

In addition to the technologies disclosed above, other technologiescould be incorporated with the scanning system of the present invention.The invention as described here may be applied to a dual-stage systemand method or, in the alternative, the processing techniques discussedherein can be applied to each of the individual scanning stages.Co-pending U.S. patent application Ser. No. 10/751,563, filed on Jan. 5,2004 and titled, “NQR Based Inspection System Using A Highly Resonantand Compact Magnetic Structure” is herein incorporated by reference inits entirety, and discloses a compact resonator probe that can be placedin proximity to shielding devices, additional resonators probes, andother components of an article screening system. Also disclosed is aresonator probe in which the number of relays or mechanical actuatorsemployed is reduced or eliminated, thereby solving many of the problemsencountered in the prior art.

NQR is a magnetic resonance technique, closely related to NuclearMagnetic Resonance (NMR), suitable for detection and/or analysis of bulkmaterials that contain a quadrupolar nucleus. Examples of such materialsare nitrogen-containing explosives such as RDX, TNT and PETN andchlorine-containing narcotics such as heroin and cocaine. Atomic nucleiwith a spin quantum number of greater than ½ and having non-sphericalelectric charge distributions possess electric quadrupole moments.Quadrupole resonance arises from the interaction of the nuclearquadrupole moment of the nucleus with the local applied electric fieldgradients produced by the surrounding atomic environment. NQR analysisfor a given material involves the irradiation of a sample that has beenplaced in a test volume with a pulsed RF magnetic field. The frequencyof the applied field used must be at or very close to one of the nuclearquadrupole resonance lines of the material under analysis. Thesefrequencies are unique to individual materials and therefore allow forvery specific identification of a material under analysis.

The exemplary NQR system, as used in either the first stage or secondstage of the present invention, employs a vane-tuned enclosed resonatorcoil design which is more compact, less susceptible to receive orgenerate radio frequency interference, has a low manufacturing cost, hasreduced flux leakage and can be placed close to other resonator probesof similar design and sensing equipments.

Referring back to FIG. 1, as described with reference to the embodimentdescribed above, a dual stage scanning system 100 comprises a housing130, which encompasses a conveyor system 115 for moving containers,baggage, luggage, or similar object 110 through a plurality of scanningstages 150, 155. The NQR system, as described below, may be in eitherstage 150 or stage 155 of the dual stage scanning system. A sensorsystem 165 is connected at the entrance to determine when an objectbeing scanned 110 enters the scan field and communicates with acontroller [not shown] to activate or deactivate an X-ray radiationsource, 170, 172, as needed. A lead lined tunnel 180 surrounds theconveyor to reduce radiation leakage outside the equipment. At least oneradiation source is not expressly depicted in FIG. 1 and would bevisible if the system were viewed from the opposite side.

The NQR system that may be employed in the dual stage scanning systemwill now be described in details with reference to the Figures. FIGS.11( a), 11(b), 11(c) and 11(d) show the front view, side view, top view,and isometric or perspective view of the resonator probe body,respectively. Referring to FIG. 11( a), the probe or basic resonator hasa rectangular or cubic volume, referred to generally as a box-likestructure 1101, and is made from a conductive material. In one preferredembodiment of the invention, this conductive material is a metal due toits relatively low resistivity. More specifically the metal will be oneof lower resistivity such as, but not limited to, silver, copper oraluminum. Copper is a preferred choice due to its high conductivity andrelatively low cost. The box-like structure 1101 is configured as anenclosed resonator probe, preferably with rectangular, orthogonal edgesfor manufacturing ease.

Referring to FIGS. 11( b) and 11(c), the box-like structure 1101 has acontinuous split 1102 around the outside perimeter. The front side andback side of the box-like structure 1101 are joined together, internallyat the middle of the structure, by a platform 1103 made from the samematerial as the box-like structure 1101. Referring to FIG. 11( d), theenclosed resonator probe 1101 has platform 1103, a top wall surface 1108parallel to platform 1103, and two inner side walls 1110, which connectthe inner top wall surface 1108 and platform 1103, forming a rectangularcutout or inspection volume 1104, through which samples to be analyzedare passed. It is preferred that the toroid of the present invention berectangular and elongated rather than rounded and circular.

FIG. 12( a) shows the layout of enclosed resonator probe 1200. In oneembodiment, enclosed resonator probe 1200 is essentially a single turntoroid fabricated, in a preferred embodiment, from copper sheets. Thetoroid of the present invention is fabricated from flat sheets of copperbent and soldered into position, creating orthogonal sides, which reduceits manufacturing cost. The tuning capacitors 1204 are provided in theprobe body, distributed along the continuous split 1202 around theperimeter of probe 1200, and are electrically connected to the probe1200 to form either a series or parallel resonant LC circuit. Theresonator probe 1200 is therefore a highly resonant compact magneticstructure.

FIG. 12( b) depicts the inspection volume 1104 of the enclosed resonatorprobe 1200 showing tuning vane 1208 housed within the hollow centralportion of enclosed resonator probe 1200 and below platform 1203. Thetuning vane 1208 comprises a conductive plate or loop 1209 mounted on apivot axle 1210 passing through the enclosed resonator probe 1200. Thepivot axle may be rotated either manually or automatically via thecontrol of a controller.

The box-like structure 1101 provides the inductive component of aresonant circuit. It is the inductance of this box combined with theapplied capacitance of tuning capacitors 1204 that determines theresonant frequency of the enclosed resonator probe 1200. Referring toFIG. 12( c), which depicts the enclosed resonator coil cross-section1205, the parallel currents that flow within the resonator probe 1200upon resonance pass from back to front across platform 1103 in thecenter and radiate across the front face of resonator probe 1200 outwardfrom the platform 1103 towards the outer perimeter (this path isdistorted in the area of the hole in the front face). The currents thenflow from the front to the back of resonator probe 1200 across the outerwalls of enclosed resonator probe 1200, subsequently passing across thedistributed capacitor 1204.

The currents pass from the outer perimeter on the back face of theresonator probe 1200 towards the center of the back face to the platform1103. This current path produces a magnetic flux path (or magnetic linesof force) 1210 around the inside of the resonator probe 1200 as shown inFIG. 12( c). The tuning capacitors 1204 distributed around thecontinuous split 1102 in the resonator probe body 1200 run parallel tothe primary magnetic flux path 1210.

The multiple parallel current paths resulting from the design ofresonator probe 1200 and distributed tuning capacitors 1204 enables theresonator probe 1200 to have a very low resistance, resulting in lowresistive losses, and therefore a very high Quality (Q) factor. Inaddition, the design of the resonator probe 1200 in the presentinvention leads to its low susceptibility to transmitting and receivingradio frequency interference or noise, and reduced flux leakage. Theresonator probe 1200 is an efficient magnetic structure with nearly allthe magnetic flux generated by the system constrained within it, furtherallowing for a high Quality (Q) factor. A high Q factor is important inthe effective performance of a resonator probe because the higher theresonator Q factor the higher the signal to noise ratio of anymeasurements made from test samples. A high Quality (Q) factor alsoleads to higher power efficiency.

The equivalent circuit diagram of the enclosed resonator probe 1200 isshown in FIG. 12( d). The resonant frequency of the enclosed resonatorprobe 1200 is changed by either altering the inductance 1214 of theresonator probe 1200 or the applied capacitance 1212 of the tuningcapacitors 1204, described above. In both cases this can be done eithercontinuously or discreetly depending on which methods are chosen.

The applied capacitance 1212 of the tuning capacitors 1204 is adjustableby use of either variable capacitors or switches which add or subtractcapacitance. A preferred method is to use a variable angle conductivevane 1208 in the flux path within the resonator probe 1200 as shown inFIGS. 12( b) and 12(c). Changing the angle of vane 1208 effectivelyalters the cross-sectional area of a segment of the resonator probe1200, interrupting the flux path 1210 within the resonator coil 1200 toa variable degree. This has the effect of changing the resonator'sinductance 1214 and therefore, its resonant frequency. The closer theangle of the tuning vane 1208 to normal (90 degrees) with respect to theflux path 1210, the greater the area of flux path 1210 intersected, thelower the inductance 1214 and therefore the higher the resonator's 1200tuned frequency. The angle of tuning vane 1208 can be changed in variousdirections provided that it is changing the amount of flux path 1210that is intersected. This method allows fine-tuning of the resonantfrequency of the resonator probe 1200. Alternatively, the inductance1214 can be adjusted by switching different sized conductive loops,which block different amounts of flux 1210. Coarse adjustment of theresonant or tuning frequency of the resonator probe 1200 is bestachieved by switching the resonant circuit's tuning capacitance 1212.

FIG. 13( a) illustrates the layout of an exemplary NQR baggage scanneras may be employed in the present invention as either a first or secondstage to detect the presence of contraband within baggage. It isparticularly effective in detecting contraband materials inconfigurations that are more difficult to detect using more establisheddetection technologies since the result of an NQR scan of a materialunder analysis depends on the number of a specific quadrupolar nucleipresent in the material, regardless of how those nuclei are distributed.The enclosed resonator probe 1200 preferably made from copper is placedwithin an outer electrically conductive electromagnetic shield (or RFshield) 1330, preferably made from, but not limited to aluminum. Theelectromagnetic shield 1330 reduces the effect of external magneticfields and also helps to constrain the generated magnetic and electricfields within the resonator probe 1200. Thus the reliability of theanalysis/detection is enhanced and resonator probe 1200 remainsessentially immune to external electromagnetic (RF) radiation. Theelectromagnetic shield 1330 also protects the external electronicapparatus from picking up electromagnetic (RF) radiation generated byresonator probe 1200. The resonator probe 1200 is tuned at NQRfrequencies of the target substance under detection or analysis. Whilethe excitation frequency need not be exactly the same as the NQRfrequency of the target substance, it is ideally within 500-1000 Hz.Tuning vane 1208 is used for fine-tuning of the resonator probe 1200.

A conveying means such as conveyor belt system 1340 is provided throughthe inspection volume 1104 in the resonator probe 1200 for transportingthe luggage through the inspection volume 1104 of the scanner. Theconveyor belt 1340 may be continually or incrementally moved via thecontrol of a controller to pass a series of samples through theresonator probe 1200. The NQR scanner is preferably encased in itsentirety in cosmetic outer panels 1320. The necessary electroniccircuits 1350 are provided for generating RF pulses, measuring the NQR,detecting suspicious baggage, activating alarms, and tuning enclosedresonator probe 1200.

In another preferred embodiment of the preferred NQR system of thepresent invention, a plurality of resonator probes can be placed inproximity to each other within the same electromagnetic shield ratherthan using a single probe coil containing a number of components fortuning to different frequencies for detecting various types ofcontraband. Each of the resonator probes, in this particular embodiment,is tuned to a different NQR frequency. In most cases, the fine-tuningfor each unit is enabled via a single fine-tuning mechanism, responsiblefor controlling the plurality of resonator probes.

FIGS. 14 and 15 depict how the resonant frequency for more than oneresonator probe can be driven from the same motor 1404, which can inturn be driven from a single control system. This method of tuningmultiple resonator probes can be extended from a minimum of tworesonator probes to as many resonators probes as desired for a givensystem depending on the requirements for the system. A significantdifference between this resonator probe configuration and other designsis the possibility of operating multiple resonator probes in closeproximity with minimal mutual interference since most flux isconstrained within the resonator coil itself.

Additionally, the nature of the resonator probe 1200 (strong magneticfields generated on the inside and magnetic fields canceling on theoutside), allows electromagnetic shielding to be placed in closeproximity to the resonator probe 1200 without disrupting the magneticfields generated inside the resonator probe. Furthermore, the design ofresonator probe 1200 is such that it is less susceptible toelectromagnetic interference, both generated and induced, therebydecreasing the need for electromagnetic shielding as compared to otherprobe designs.

The NQR security system of the present invention, as described herein,may be applied to a dual-stage scanning system and method as describedabove, or, in the alternative the processing techniques discussed hereincan be applied to each of the individual scanning stages. Specifically,the NQR security system can be employed as one of a plurality ofscanning stages with the other stages being of the types discussedherein, such as X-ray scatter or X-ray transmission. Alternative the NQRsecurity system can be incorporated into a single stage with or withoutadditional technologies, such as X-ray scatter or X-ray transmission. Inaddition to the technologies disclosed above, other technologies couldbe incorporated with the scanning system of the present invention.

b. An Exemplary Microwave Metal Detection and Imaging System

In another preferred embodiment, the present invention is directedtoward a microwave metal detection and imaging system that uses multipletransmitter/receiver pairs (also referred to as transmit/receive pairs)of microwave antennas to measure the presence of conductive materialappearing in the space between each transmit/receive pair. Each antennapair has an ideal primary transmission path, which is effectively theshortest route between the two antennas. Conductive material will blockor attenuate the transmission between the antenna pairs if it is placedin the direct transmission path (the space between each transmit/receivepair).

Transmit/receive pairs are arranged in a linear array. The object to bescanned passes through such transmit/receive pairs (for example, bymeans of a conveyor belt), creating a two-dimensional image ofconductive items concealed within an object under inspection. Byrepeating this process for each dimension (in a three dimensionalstructure, the dimensions run along each of an x-axis, a y-axis, and az-axis, whereby the conveyor belt runs along the x-axis and imagepatterns are obtained in the y and z-axes), it becomes possible toestimate the volume of conductive items/objects. An appropriate designof antenna rays enables the measurement of the metallic or conductivecontent of items concealed within a three-dimensional object, in eachphysical dimension, while the object under examination moves in only onedirection (for example, along the x-axis, as on a conveyor belt).

The transmit/receive pairs are designed to prevent the multiple pairs inthe array from interfering with one another. Furthermore, if the pairsare not appropriately designed, multiple microwave reflection paths arepossible, which can result in confusing data being developed fromtransmission paths other than the most direct transmission path, i.e.between designated transmit/receive pairs. This problem is solved by aunique combination of the system layout, proper material selection, andantenna design.

Referring now to FIG. 16, the most direct transmission pathways oftransmit/receive pairs 1630 as in the present invention are depicted.Transmitter antenna 1640 and receiver antenna 1650 comprisetransmit/receive pair 1630. In microwave imaging system 1600, microwavestransmit, to a large extent, directly from a transmitter antenna 1640 toits corresponding receiver antenna 1650 in a straight line path, therebyforming microwave beams 1610.

The microwave frequency and amplitude is selected in a suitable range toallow for discrimination of conductive and non-conductive items and suchthat transmission is possible through normal non-conductive luggagematerials. The preferred amplitude of the transmitted signal is atsignificantly above the noise floor such that measurements are a clearindication of the presence of conductive/non-conductive items and arenot determined by fluctuations in microwave noise. The frequency ofoperation is such that penetration is sufficient for the detection andimaging of objects within typical packages and bags yet with a minimalamount of inaccuracies introduced due to items with high dielectricloss. In one embodiment, the frequencies utilized are in the range of 1to 30 GHz and, more particularly, in the range of 3-12 GHz. Theamplitude should be sufficient to generate a signal to noise ratio ofpreferably at least 4 after traveling through normal, non-metallicclutter found in typical baggage.

Non-conductive materials should largely allow the microwaves to passrelatively un-attenuated. However, the introduction of a conductiveobject under inspection 1620 between the transmit/receive pair willsubstantially attenuate the transmission of microwaves. This isparticularly true if conductive object 1620 is metallic.

c. Microwave Imaging as a Single Stage System

In a preferred embodiment of the present invention, the object underinspection is passed through a conveyor belt system 1700, as shown inFIG. 17. FIG. 17 is an exemplary imaging system that would use conveyorbelt 1710 to progress the object under inspection through one or moretransmit/receive pairs of microwave antenna array(s) 1720. As conveyorbelt 1710 passes an object under inspection through the one or moremicrowave antenna array(s) 1720, a record of the attenuation betweeneach transmit/receive pair within microwave antenna array(s) 1720 iskept for varying positions of conveyor belt 1710. The record is directlyrelated to the contents of the object under inspection and an image ofattenuation levels observed within the object under inspection can bedisplayed.

To keep the transmission path between distinct transmitter receiverantenna pairs separate and to prevent cross talk between channels, eachtransmit/receive antenna pair could be frequency multiplexed, timemultiplexed or have its transmit signal uniquely coded using one ofvarious modulation schemes as known to those of skill in the art. Thesetechniques could be used either alone or in combination to allowinformation for transmit/receive channels to remain distinct and areused in conjunction with careful antenna design and system layout tokeep inherent crosstalk between channels to an acceptably low level.

In one embodiment, crosstalk is minimized by using high gain,directional antennas. In another embodiment, crosstalk is minimized bylaying out antennas such that any sidelobe patterns do not intersect,i.e. alternating the orientation of adjacent antennas such that theradiation patterns between adjacent antenna pairs are rotated through90°. In another embodiment, crosstalk is minimized by selecting anantenna pair with different polarizations to their neighbors, i.e. thefirst pair is horizontally polarized while the second pair is verticallypolarized or the first pair is circularly polarized clockwise while thesecond pair is circularly polarized anti-clockwise. In anotherembodiment, crosstalk is minimized by shielding antennas from theirneighbors using combinations of shielding and/or RF energy absorbingmaterials around each antenna.

As shown in FIG. 18, if multiple antenna arrays are used in more thanone axis a calculation of the volume and shape of conductive items canbe performed. This information can be used as part of an automateddecision making process to determine whether a conductive item has theproperties of a pre-determined threat type. In a preferred embodiment,the multiple antenna arrays are depicted as on both the y-axis andz-axis. The motion along the conveyor belt system is represented as anx-axis movement, while the multiple antenna arrays are represented asy-axis or z-axis measurements, with y-axis transmitter antennas matchingto y-axis receiver antennas (i.e., Y-Rx1 aligned with Y-Tx1 upto Y-Rx13aligned with Y-Tx13) and with z-axis transmitter antennas matching toz-axis receiver antennas (i.e., Z-Rx1 aligned with Z-Tx1 upto Z-Rx13aligned with Z-Tx13). This calculation can be performed if the data fromthe various antenna pairs is passed to a computer via a suitableacquisition system, as are well known to those of ordinary skill in theart.

d. Microwave Imaging as Used in a Dual Stage System

As an alternate embodiment to using the microwave metal detection andimaging system of the present invention in a single stage, the microwaveimaging system as described in the previous section may be employed as afirst stage in the aforementioned dual stage scanning system. Themicrowave imaging system as the first stage can direct the scanning ofthe second stage.

The microwave imaging system of the present invention may be used inconjunction with and/or housed within a security system that uses one ormore additional technologies to detect potential threats, including, butnot limited to X-ray imaging systems, CT systems, x-ray diffractionsystems, NQR systems, PFNA systems, TNA systems and explosive tracedetection systems. Most or all of these systems can be incorporated intoa conveyorised system. In typical systems, the conveyor will run througha tunnel. The tunnel may function as an RF shield in the case of an NQRsystem or as X-ray shielding in the case of an X-ray system, or both,but is not limited to such usage. Microwave imaging technology can beapplied such that it is capable of operating within enclosed metallictunnels. Metallic tunnels are typical of the architecture of X-ray, CTand NQR screening systems and hence the addition of shield detection, asin the microwave imaging system of the present invention, can be readilyincorporated into the housing of a complimentary screening technology.

In an exemplary embodiment and referring back to FIG. 1, a dual stagescanning system 100 comprises a housing 130, which encompasses aconveyor system 115 for moving containers, baggage, luggage, or similarobject 110 through a plurality of scanning stages 150, 155. In suchpreferred embodiment, the microwave metal detection and imaging systemoccupies first scanning stage 150. A sensor system 165 is connected atthe entrance to determine when an object being scanned 110 enters thescan field and communicates with a controller [not shown] to activate ordeactivate an X-ray radiation source, 170, 172, as needed. A lead linedtunnel 180 surrounds the conveyor to reduce radiation leakage outsidethe equipment. At least one radiation source is not expressly depictedin FIG. 1 and would be visible if the system were viewed from theopposite side.

e. X-Ray Transmission Combined with Microwave (Single Stage)

In another preferred embodiment, FIG. 19 illustrates an example of howthe microwave imaging system of the present invention may be used incombination with an x-ray imaging system to overlay the image of thin(and therefore having low x-ray attenuation) conductive items that arenot readily detectable using x-rays. The x-ray imaging system is thusenhanced by the ability to show a x-ray attenuation image combined withan electrical conductivity image (produced by the microwave system).

In a preferred embodiment, as shown in FIG. 19, the microwave imagingsystem 1900, comprised of transmit/receive antenna arrays 1910, is builtinto tunnel 1920, which would have a dual or multiple purpose (exemplarymultiple purpose imaging machines are described further below) withinthe security system, and may include x-ray shielding or RF noiseshielding for NQR security systems as described in detail above. Thehousing for the microwave imaging system would be small and wouldconsist of opening 1930 in tunnel 1920 into which transmit/receiveantenna arrays 1910 could be housed. Tunnel 1920 is lined with amaterial which does not attenuate microwaves significantly and allowsfor the inner walls of the tunnel to be smooth. The pitch betweenantenna pairs would be a function of the microwave frequency used alongwith the image resolution desired. This, in large part, depends on theobject under inspection producing such image, and can include, but isnot limited to baggage, carry on luggage, cargo or mail as well as thethreat items that are being searched for during the inspection. Whilethe object under inspection is passed through the security system viaconveyor belt 1940, microwave image 1950 and X-ray image 1960 areproduced. By using software processing techniques, microwave image 1950and X-ray image 1960 are combined to produce combined image 1970.

In one embodiment, images are combined by displaying the X-ray image andvisually defining the area containing metal using a translucent boxhaving a different color or a different shade relative to the X-rayimage. In another embodiment, images are combined by visually definingthe area containing metal by drawing a plurality of lines around theperimeter of the detected metal volume, while retaining the x-ray image.

f. NQR Combined with Microwave

In an exemplary embodiment, the present invention employs a carefullydesigned microwave imaging system that is compatible with NuclearQuadrupole Resonance (NQR) detection systems. NQR has shown significantpotential for the detection of a range of materials, particularly, asdescribed above, for the detection of the types of explosives that canbe the most challenging to detect using x-rays or CT machines. Onepotential weakness of NQR, however, is that with carefully designedelectromagnetic shielding the materials which the NQR technique is beingused to detect can be rendered undetectable. The shielding effect can bemitigated by the fact that such shielding consists of conductive(typically metal) volumes which must completely encapsulate the item tobe detected in order to mask it. Highly conductive material (forexample, metal) can be used to enclose explosives and thus render theexplosives undetectable using NQR.

In conventional systems, because the items being searched for aretypically larger in size compared to most metal clutter, (i.e. keys,coins, zippers, etc) the counter measure can be detected using a varietyof metal detection techniques. The presence of a conductive loop aroundmuch luggage, however, means that the simplest forms of inductive metaldetectors would have limited performance.

The microwave imaging system of the present invention may beincorporated into the NQR system as a shield detection system. FIG. 20is an isometric sketch of the microwave imaging system described abovewith respect to FIG. 17, which can be housed in metallic tunnel 2000that can also function as either the waveguide for an NQR system asdescribed above or as the tunnel of an X-Ray/CT scan imaging machine.Metallic tunnel housing 2000 can be employed to save space required fora multi-technology system. In one embodiment, as shown in FIG. 20,housing for microwave imaging system antenna pair 2010 is arranged in acutaway portion of metallic tunnel 2000. The microwave imaging systemuses arrays of transmit/receive antenna pairs arranged around aconveyorised system to measure the conductive content of objects passingon a conveyor or equivalent transport mechanism.

Again referring to FIG. 20, the exemplary NQR security system asdescribed with respect to FIGS. 11( a)-11(d) can comprise metallictunnel 2000. The housing for microwave imaging system antennas 2010 isincorporated into this structure. The transmit/receive antenna pairs maybe recessed within the metallic tunnels which are inherent to thestructure of other detection technologies

g. Other Detection Systems Combined with Microwave

If the information from a conductive volume detecting system is combinedwith the result of a NQR scan, a result can be generated after the scanstating that the object under inspection is either clear, triggered aNQR alarm, or has areas that can be detected but not thoroughlyinspected/interrogated. In the event that a detected area cannot beinterrogated, the positional information for the shielded area can betransmitted to an imaging system (for example, but not limited to, CT orx-ray), which can direct the attention of an operator (or focusautomated detection techniques) to the area that cannot be screenedusing NQR. This could be done by, for example, overlaying the metalimage on the x-ray image.

In another preferred embodiment of the present invention, a microwaveimaging system is incorporated into an integrated multi-technologysystem, comprised of the NQR system described above and including, butnot limited to imaging systems such as CT scanners and X-ray scanners.FIG. 21 depicts an example of an integrated multi-technology system. Aline-scan X-ray system 2100, comprising X-ray generator 2102 and afolded array of L-shaped X-ray detectors 2104 is integrated with thedual coil NQR baggage scanner. Unlike conventional X-ray baggagescanners, NQR based baggage scanners only detect the presence ofcontraband in baggage without revealing their exact location in thebaggage. Thus by integrating X-ray system 2100 with the NQR basedscanner, the integrated scanning system will also be able to locate thecontraband in the baggage. By further integrating the microwave metaldetection and scanning system of the present invention into the NQRbaggage scanning system, a shield detection system is incorporated, thusensuring complete scanning of the object under inspection.

A line-scan X-ray system 2100 is provided in between the two resonatorprobes 1200 a and 1200 b. The fan shaped X-ray beams generated fromX-ray generator 2102 scans the luggage passing through inspection volume1104 on conveyor belt 1340 and impinges the X-ray detector 2104. Thesystem is equipped with an alarm circuit, which will activate uponsuspicion.

To allow CT scanners and/or X-ray scanners to be closely integrated to aproduce a multi-technology system, it may be necessary to keep the X-rayor CT equipment outside of the electromagnetic shield used inconjunction with the resonator probe(s). Referring to FIG. 22, resonatorprobes 1200 a and 1200 b are surrounded by an electromagnetic shield1502. The X-ray scanner 2100, having detectors 2104 and a X-ray source2102, emits X-ray radiation. In order for the X-rays 2105 used by the CTor X-ray scanner 2100 to be allowed to pass through the electromagneticshielding 1502 relatively un-attenuated, it is preferred that theshielding 2200 through which the X-rays 2105 are expected to pass ismade of high conductivity material that is sufficiently thin and/or of alow density. The portion of electromagnetic shielding 2200 which offerslow attenuation to X-rays could be integral to the rest of the shield orbe an insert or inserts of thinner high conductivity shielding materialsuch as aluminum which would minimally interrupt the x-ray beam 2105between X-ray source 2102 and X-ray detectors 2104.

h. Further Examined by X-Ray Diffraction, Etc.

The inventions and embodiments thereof described here deal with amicrowave imaging system, which can be used alone or in combination withone or a plurality of additional detection systems, allowing for theprovision of three-dimensional positional information which can betransmitted to complementary detection sensors targeted at volumeswithin an object that cannot be screened effectively using NQR. Inaddition, a further use of the information from the conductive volumedetecting system is to automatically direct material specific detectionsensors which are not prone to masking by metallic shielding to theshielded volume. Examples of this type of technology include X-raydiffraction, thermal neutron analysis and pulsed fast neutron analysis.

i. Presentation of Imaging Information

In another preferred embodiment of combined microwave and X-ray imagingsystems, FIG. 23 depicts overlaid microwave and X-ray images. A singlebroad beam transmit antenna 2310 transmits microwave beam paths 2320 ina fan or cone-shaped radiation pattern and thus illuminates multiplereceive antennas 2330. The position of the transmit antenna and thearray of receive antennas can be such that it is similar to that of thex-ray generator and folded array of x-ray detectors in an x-ray imagingsystem. The layout, antenna type and switching order of transmit/receivepairs can be configured such that the microwave imaging system has thesame geometry as the X-ray imaging system to which it is coupled,allowing for simple and accurate overlay.

FIG. 24 illustrates yet another preferred embodiment of the presentinvention whereby the microwave imaging system is used to generate acomputed tomography (CT) compatible image. In this embodiment, eachtransmit antenna can be activated in turn. Both the transmit antennaarray 2410 and receive antenna array 2420 extend around the fullperimeter of the system inspection tunnel 2400. Transmit antennas 2410and receive antennas 2420 may be parallel and placed side-by-side,concentric, or any other similar geometry. It is also possible that eachantenna may be used for both transmitting and receiving and switchbetween the two at appropriate times. In the preferred embodiment, asingle antenna transmits a wide beam at any given time, therebyilluminating multiple receive antennas 2420. Transmit antennas 2410sequentially switch on around the perimeter of tunnel 2400, thus,transmission takes place from each possible position around inspectiontunnel 2400. This method allows for multiple transmission images for agiven object to be generated at each angle. The received data from thereceive antennas is processed for each transmission position creating acomputed tomography image of the conductive contents of items underinspection. The information in theses images can be processed to developcomputed tomography slices accurately showing the size, shape andposition of conductive items within the inspection volume.

The above examples are merely illustrative of the many applications ofthe system of present invention. Although only a few embodiments of thepresent invention have been described herein, it should be understoodthat the present invention might be embodied in many other specificforms without departing from the spirit or scope of the invention. Forexample, while dual-stage scanning systems have been described withreference to first stage scanning systems comprising a dual-view linescanner and complimentary second stage scanning systems, comprising atransmission and scatter scan, other modifications and changes can bemade by those of ordinary skill in the art. Additionally, while many ofthe systems described herein have been described with respect to use indual stage scanning systems, it is to be understood that the embodimentsdescribed herein may be used as single stage scanning systems.Therefore, the present examples and embodiments are to be considered asillustrative and not restrictive, and the invention may be modifiedwithin the scope of the appended claims.

We claim:
 1. A method of scanning an object, comprising the steps of:subjecting said object to a first screening system comprising microwavearrays, wherein said microwave arrays comprise more than one microwavetransmitter and more than one microwave receiver, wherein the microwavetransmitters and microwave receivers are separated by an inspectionregion, and wherein each of said microwave transmitters transmitsmicrowave radiation to more than one microwave receivers and wherein thefirst screening system comprises a data acquisition system that acquiresdata from said microwave receivers, said data comprising microwaveradiation absorption data; and subjecting said object to a secondscreening system selected from any one of NQR-based screening, X-raytransmission based screening, X-ray scattered based screening, orComputed Tomography based screening.
 2. The method of claim 1 whereinsaid first screening system operates concurrent with said secondscreening system.
 3. The method of claim 1 wherein said first screeningsystem operates serially with respect to said second screening system.4. The method of claim 1 wherein said microwave transmitter emitscontrollably directed microwave radiation toward an object underinspection wherein said object under inspection absorbs radiation in amanner dependent upon its metal content.
 5. The method of claim 4wherein said microwave radiation absorption data can be used to generatea measurement of metal content.
 6. The method of claim 5 wherein theobject under inspection is selected for screening by said secondscreening system if the measurement is different than a pre-definedvalue.
 7. The method of claim 5 wherein the measurement can be comparedto a plurality of predefined threats.
 8. The method of claim 5 whereinthe object under inspection is ignored by a system operator if themeasurement is different than a pre-defined value.
 9. The method ofclaim 5 wherein the measurement is used to generate a microwave image.10. The method of claim 5 wherein the measurement is used to generate amicrowave image and the microwave image is combined with an imageproduced by said second screening system.
 11. The method of claim 5wherein the measurement is used to generate positional information ofmetal content in the object under inspection.
 12. The method of claim 11wherein the positional information of metal content is used to direct ananalysis from material specific detection technology.
 13. The method ofclaim 12 wherein said material specific detection technology is selectedfrom any one of x-ray diffraction, thermal neutron analysis or pulsedfast neutron analysis.
 14. The method of claim 1 wherein the microwavetransmitters and microwave receivers are configured in a manner thatreplicates X-ray beam fan beam geometry.
 15. The method of claim 1wherein the microwave transmitters and microwave receivers areconfigured in a manner that replicates X-ray beam folded array geometry.16. The method of claim 1 wherein the microwave transmitters andmicrowave receivers are configured in a manner that replicates ComputedTomography array geometry.
 17. The method of claim 1 wherein themicrowave transmitters are broad beam transmit antennas.
 18. The methodof claim 1 wherein the microwave receivers are narrow band receiveantennas.
 19. The method of claim 1 wherein said broad beam transmitantennas are configured in parallel with said narrow band receiveantennas and switched such that each transmit antenna transmits toseveral receive antennas.
 20. The method of claim 1 wherein saidswitching occurs to move an illumination point around a region.